My name is ***** ***** I am an experienced criminal
Is your fiance currently being detained on the Costa Rica warrant?
The US and Costa Rica have a formal and detailed extradition treaty between them, in which both countries agree to cooperate in extradition procedures if the offense involved is one for which the alleged offender could face more than a year in jail.
You can read the extradition treaty with Costa Rica here. It loaded slowly for me, so give it time:
If his offense in Costa Rica qualifies as extraditable and Costa Rica can demonstrate that and can provide the required documentation, the US will indeed extradite him. If the authorities in California also have probable cause
to believe that the $3,000 was either the fruit or the means of a crime, they will confiscate his money. I would argue that the fact of a warrant does not immediately render your fiance's money contraband, but that doesn't mean that the authorities can't take it and he may have to retain counsel to try to establish the origin and the purpose for the case so that it's not forfeited.